1The fact or condition of being born out of wedlock; illegitimacy. Also figurative: the state or condition of being mixed, adulterated, or debased; heterogeneity.
2 slang The quality or state of being unpleasant, objectionable, or annoying; behaviour characteristic of a bastard. Often with modifying adjective.
Late 16th century; earliest use found in Thomas Nashe (d. c1601), writer. From bastard + -ism.